Motor starter/relay sizing question

I just ran your motor parameters through my new free online motor circuit calculator at

and the results came out ok for 3/4 and 1 hp. However you did not give the horsepower, but it looks like you have either a 3/4 hp or 1 hp motor based on locked rotor current using Table 430.251(A). The output page is quite large so I am not copying it here. If the output from the module is a Class 2 power limited circuit (which it probably is) then you will have to keep it separated from the power wiring as covered in article 725. If it is Class 2 use a 16/2 or

18/2 shielded cable and ground the shield at only one end. You can buy motor controllers (starters) with 24 volt DC coils but this is not the normal method of controlling a motor starter or controller. Instead use a 24 volt DC Potter Brumfield relay and wire the motor controller with a 120 volt coil through the contacts. Control the PB relay with your signal. I am not an electrical engineer, but am a licensed electrical administrator in Alaska and licensed electrician wireman in Alaska and Washington that plugs his site here from time to time.

Yes, sorry, I completely forgot the horsepower (3/4 hp). I'll check out your website, and into the info you provided.

Thank you.

If you have the money in the budget, an Allen Bradley Variable Frequency Drive is really a great way to go. They are basically a programmable motor controller that can be interfaced with your computer network. They are also very reliable and are now common in industrial locations. They can be used for on off control (or proportional control) and will take you signal directly as an input. They come with an instruction book that is sufficient for learning how to do the programming after about 10 hours of study. The future use of these VFD's is immense since they have about 350 programmable variables. And I don't work for AB, but have installed and programmed their VFD's. I was impressed. As I recall, they are a little spendy.

WRONG! He has a 1 phase motor! VFDs are only for 3 phase motors.

Just buy a relay that has contacts rated for starting a 3/4HP motor. it will be larger than the plug-in style plastic case relays typically called "ice cube relays", because they are only rated for 1/2HP. 3/4HP will mean a slightly larger one, but still very cheap. Most 1 phase motors like that are already internally protected so you will not need any other protective device other than a fuse or circuit breaker sized for the wire going to the motor.

Also check out the relay coil "inrush current" when you find one, and compare that to the amp rating of the output you have. If the relay draws too much for that output, you may need to use another small relay to drive the larger one, a systems called an "interposing relay".

Sorry, didn't have your output info available when responding to the other posting, but at 2A capacity, you will be able to drive a 3/4HP rated relay no problem, so forget about the interposing relay business.

Here's a relay like what you need. A bit of overkill, but for $7.24 it's worth it.

?SKU=09B8542&N=0

Where have you been? There are a ton of Single phase VFD's available. Just search single phase VFD at Google.

Also, I seriously doubt the ice cube relay design or ice cube relay with "an interposing relay" that you refer to can handle the locked rotor current, inrush current and fault current without a detailed analysis. The original question was posted by a Mechanical Engineer that admits he has limited experience and who probably is unfamiliar with fault current. The safe and simple approach, in this case, appears to be a motor starter combination with a ground-fault and short circuit protector and controller with overload relays with an auxiliary relay driven by a Class 2 DC signal located in a separate box that we often install in industrial locations. A LISTED motor starter for 3/4 hp motors usually can handle the fault current, inrush current, and locked rotor current without further considerations. It makes things simple and provides a guaranteed high degree of reliability which I think is of practical importance for the mechanical engineer. Like I said before, I am not an engineer, but do have considerable experience installing motor circuits that have been designed by very experienced PE's.

I will give you credit on one item. The VFD's outputs are three phase, but the input can be single phase. So we were both half right on that one. The literature online for the AB Powerflex 40 is misleading in that it states that the VFD can be used for 120, 230, 460 volts but it does not state the phases so I called Allen Bradley at 440-646-5800 and asked them. Apparently, there is a harmonic problem with single phase outputs that has not been solved yet. I have installed only three phase VFD's and relied on the online material for the single phase use

- my error.

The Magnecraft relay has a rating of 3HP at 240V. It should not have a problem with locked rotor (inrush?) current. I have not seen ground fault protection on a 3/4HP motor. The Magnecraft lit does not indicate a value for short circuit protection. A 5.4A motor could use a 15 or 20A branch circuit breaker. The relay does not provide overload protection for the motor. If the motor is marked "Thermally protected" that is adequate, else something else would be needed. The relay is a UL Recognized Component, indicating it is intended to be used in a manufactured device. That may or may not indicate a limitation for this application. If this is hardwired, an inspector would want a UL listed relay with a HP rating (if the motor is thrermally protected) or a motor starter (which has overload protection). "NEMA" sized motor starters would last longer if this is a long term application. For a single application use for a plug in device for noncritical application the relay is should be adequate.

A disconnect is also required. That could be a 2 pole switch rated 3/4 HP @240V. Wall switches rated 15 or 20A would work.

bud--